Characterization of Sagittal Spine Alignment With Reference to the Gravity Line and Vertebral Slopes

Mini Spinal sagittal realignment necessitates a reference posture, and thus far this has only been defined for an “averaged” curve via horizontal offsets from the gravity line (GL). This prospective study of 169 healthy subjects demonstrates normative sagittal spatial orientation of each vertebra, for all Roussouly curve types, using vertebral slopes and horizontal offsets from the GL. Study Design. Prospective study. Objective. To map the healthy standing alignment of the adult spine, grouped according to Roussouly curve types, using both horizontal offset distance from the gravity line (GL), as well as vertebral slope measurements. Summary of Background Data. Spinal sagittal realignment requires a reference posture, and this has been defined in the literature via horizontal offsets from the GL. While useful, this does not provide information on the orientation of each vertebral segment, or distinguish between the various physiological curve types. Methods. A total of 169 consecutive young adult subjects with healthy spines were recruited over a year. (EOS Imaging, Paris, France) whole body radiographs were performed. Radiographic measurements collected included sagittal vertical axis (SVA), T1-slope, global cervical angle (GCA), global thoracic angle (GTA), global lumbar angle (GLA), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), apical and inflection vertebrae. Outcome measures comprised slope measurements, and distance offsets relative to the GL for individual vertebrae from T3 to S1. Results. GLA, PI, PT, SS, apex of lordosis, and inflection vertebra were significantly different across groups, while SVA, T1-slope, GCA, GTA, and apex of kyphosis were not. Mean PI to LL discrepancy for Type I to IV groups were 8.0°, 2.3°, 4.8°, and 3.0°, respectively. Between groups, T3, T7 to T12, and L2 to S1 slopes and T9 to L3 offset distances from GL were significantly different, while the distance of the hip center from the GL was not. GLA was significantly different between curve types except between Type 1 and 2 curves, while the inflection vertebrae were not significantly different between Type 1 and 2 curves, as well as Type 3 and 4 curves. Conclusion. This study demonstrates normative sagittal spatial orientation of each vertebra in healthy adults, for each Roussouly type. Comparison across groups suggests the possibility of further refining the sagittal curve patterns described by Roussouly. Level of Evidence: 3